The permeability of the neuroependyma between CSF and brain extracellular space has been studied in fetal sheep of 60 and 125 days gestation. Both radioactive ([3H]inulin, [14C]sucrose, [125I]albumin) and visible (horseradish peroxidase) markers have been perfused through the ventricular system for periods of up to 5 h in anaesthetized exteriorized fetal sheep whose physiological condition was monitored continuously. A previously undescribed barrier between CSF and brain extracellular fluid has been discovered in the immature (60-day) fetal sheep. Horseradish peroxidase penetration was confined to a limited depth of the neuroependyma and was mainly into the cells lining the cerebral ventricles; in older fetuses there was extracellular penetration to a distance of several millimetres from the ventricular surface, as previously described in adult animals. The volumes of distribution of sucrose and insulin were less in the immature brain than in the more mature brain, which may be a reflection of restricted diffusion across the neuroependyma in the younger brains. The morphological nature of the barrier in fetuses of 60 days and younger appears to be a membrane specialization between the cells of the neuroependyma. It is of a type not previously described; it seems to have the effect of narrowing rather than obliterating the extracellular pathway between CSF and brain. The possible functional significance of this observation is discussed.